Steve
I replaced the tanks in a Sedan 36 in about 2002.
We took the tanks out but the engines were out for replacement anyway.
IIRC the engine on that side would need to come out to remove the tank. If you dismantled all the woodwork beneath the windows and the floor bearing structure over the engines you might be able to squeeze them out but it would be a real pain to do.
The original tanks in mine were mild steel 550 litres each and the tops had corroded through with salt from the engine room air inlets.
I replaced them with mild steel tanks painted, of about 875 litres each with a sump on the bottom of about 2 litres with a drain valve and lock on so water and crap can be removed.
Thanks all. on advice received, it seems more unlikely now that the problem is actually with the tank itself. More investigation needed. However, taking account the noted significant difficulties in removing fuel tanks with engines in, should/when the need to replace the fuel tanks arises, it seems clear that the best option would be to remove engines for clear access.